1. Field of the Invention
The present invention relates generally to a material of a gate electrode of a field effect semiconductor device (hereinafter, referred to as "FET"), and more particularly to a Schottky gate electrode structure of a GaAs FET.
2. Description of the Related Art
A pellet of a conventional GaAs FET, for example, a GaAs Schottky gate field effect transistor (hereinafter, referred to as "MESFET") has a cross section as shown in FIG. 8. An Al layer 4 having a thickness of about 2 nm is formed, by means of an evaporating or sputtering method, on the surface of an N-type low-concentration impurity region (N.sup.- type channel region) 2 that is formed in a GaAs semi-insulating substrate 1, and a W (tungsten) film 5 is then formed on the Al layer 4 by means of an evaporating or sputtering method thereby providing a gate electrode 5. Reference numeral 3 indicates source and drain regions which are formed such that, after a desired impurity is ion-implanted in the substrate 1 by a self-alignment process with use of the gate electrode 5 as a mask, the substrate 1 is subjected to a heat treatment. Numeral 6 denotes source and drain electrodes which are in ohmic contact with the N.sup.30 -type source and drain regions 3.
It is necessary that the gate electrode be formed of a material which does not deteriorate electrical characteristics of a Schottky junction, even under a high temperature heat treatment carried out after the ion implantation. Thus, refractory metal base compounds are used as the gate electrode materials. As heat-resistant gate electrode materials, materials such as WSix, WNx, TiW, TiWNx, and the like are well known.
The conventional refractory electrode materials have a bad adhesion to GaAs substrate. The gate electrode formed of such materials may be peeled off after the implanted layer is activated by the heat treatment. Also, if the thickness of an electrode layer is increased to obtain lower resistance, the electrode layer may be peeled off due to an internal stress of the material itself. Therefore, it is difficult to obtain low resistance of the electrode layer by increasing it moreover, the limit value of heat resisting temperature for Schottky characteristics is about 800.degree. C.
As has been described above, it is imperative that the gate electrode of a GaAs FET be formed of a material which does not deteriorate Schottky characteristics (e.g., a Schottky barrier height .phi.B, an ideality factor n-value, etc.) due to the high-temperature heat treatment during the manufacturing process. Also, the gate electrode must be formed of a material having physically high adhesion to GaAs substrate. In the prior art, however, the gate electrode is often peeled off and Schottky characteristics are deteriorated due to the heat treatment (about 800.degree. C), e.g., for activating the source and drain regions, which is performed after the gate electrode has been formed.